This invention relates to tags for use in article surveillance systems and, in particular, to tags capable of being remotely disabled or deactivated and capable of exhibiting a unique signature.
One form of tag employed in present electronic article surveillance systems utilizes a high Q resonant inductor (L) -capacitor (C) circuit. In systems using this type of tag, typically a transmitter repetitively projects a swept RF field into a surveillance zone which is monitored by a receiver.
When an article carrying the resonant tag is placed in the surveillance zone, the tag causes a perturbation in the swept RF field when the frequency of the RF field approaches the resonant frequency of the tag. This perturbation is detected by the system receiver which activates various alarms, or other appropriate signals, to indicate the presence of the tag and, therefore, the article in the zone.
Since detection of a resonant tag is based upon receiving perturbations at a resonant frequency expected by the receiver, changing the resonant frequency of the tag effectively deactivates the tag. A variety of deactivating techniques for changing or altering the resonant frequency of a resonant tag have been used. In U.S. Pat. No. 4,063,229, issued on Dec. 13, 1977, to John Welsh and Richard N. Vaughn for "Article Surveillance", and assigned to the same assignee hereof, there is described a tag containing a semiconductor diode. To deactivate the tag, the semiconductor diode is burnt out by a relatively high power RF field which is inductively coupled to the tag. In U.S. Pat. No. 4,021,705, issued May 3, 1977, to George Jay Lichtblau for "Resonant Tag Circuits Having One Or More Fusible Links", there is described a resonant tag having one or more fusible links for altering the characteristics of the circuit. Each fusible link is able to be fused by a radiated high energy RF field of a predetermined frequency. The fusing of a fusible link changes the value of the inductance of the tag, thereby changing the resonant frequency and deactivating the tag.
Both of the aforesaid deactivation techniques require the use of a high energy RF field which may not be desirable in many applications. In U.S. Pat. No. 4,318,090, issued Mar. 2, 1982, to Douglas A. Narlow and Eugene Stevens for "Apparatus For Deactivating A Surveillance Tag", and also assigned to the same assignee hereof, there is described a wand like probe which contacts terminals on a resonant tag. The wand applies a low energy current through a diode of the tag, thereby destroying its unidirectional characteristics and changing the resonant characteristic of the tag. While the wand alleviates the need to use a high energy RF field, the wand can not be used to remotely deactivate the tag.
A further limitation of the above described resonant tags is that they are not capable of being restored to an active state after being deactivated. Therefore, a tag, upon deactivation, may not be used again.
The resonance effect exhibited by a tag can, in certain instances, occur in ordinary objects. Therefore, certain ordinary objects, placed within the surveillance zone, will cause perturbations in the RF field similar to those caused by resonant tags, thereby, resulting in a false alarm. This effect can be minimized by decreasing the range of frequencies over which the receiver initiates an alarm. However, this requires that the resonant frequency of each tag be more tightly controlled. To control the resonant frequency, high tolerance components and/or precision manufacturing techniques must be employed, thereby increasing the cost per tag.
It is, therefore, a primary object of the present invention to provide an improved resonant tag.
It is a further object of the present invention to provide a resonant tag that can be remotely deactivated by a low energy field.
It is still a further object of the present invention to provide a resonant tag that has a unique signature not readily reproduced in ordinary objects.
It is yet a further object of the present invention to provide a resonant tag having a signature which can be used as a code.